This invention relates to a resonant circuit including a strip member and more particularly to a resonant circuit utilizing a strip member formed in a U-shape.
A recent trend is toward application of a microstrip line structure to design a smaller solid-state circuitry for a UHF television tuner. To cite an instance, the U.S. Pat. No. 3,659,205 for "A VARACTOR TUNED MICROSTRIP TUNER" sets forth microstrip members formed in the "L", "T" and "U" shapes for use with a television tuner. As described in the specification of said United States patent, the "U" shaped microstrip member itself does not act as a resonance line, but is used simply for coupling resonant lines lying on both sides of said microstrip member. Application of a microstrip member as a resonant circuit generally raises problems in respect of an output and impedance. FIG. 1 shows an oscillator circuit formed of a resonant circuit including the prior art microstrip line for use with a UHF television tuner. A resonant circuit 10 included in said oscillator circuit is a shortened quarter wavelength type. This resonant circuit 10 is formed of as I shaped distributed line 11 and lumped capacitor 12 connected in series. Further, the resonant circuit 10 is connected between the opposite ground-contacting portions 13, 14 of the oscillating circuit and is also disposed within a feedback loop of a signal oscillating section 15. An output signal from the signal oscillating section 15 passes through an L-shaped output circuit 16 coupled to the resonant circuit 10 and is issued from an output terminal 17.
According to the arrangement of said oscillation, impedance occurring in the ground-contacting portions 13, 14 surrounding the signal-oscillating section 15 is equivalently applied in series to both ends of a series circuit formed of the distribued line 11 and lumped capacitor 12. The equivalent introduction of the impedance leads to prominent loses of energy charged into and discharged from the lumped capacitor 12, and in consequence a decreased output power from the signal oscillating section 15. Further, an inductance between the opposite ground-contacting portions 13, 14 and a floating capacitance appearing in the ground-contacting portion 13 give rise to parasitic oscillations which are repeated at a different frequency from the resonance frequency of the resonance circuit 10 which is defined by an inductance in the distributed line 11 and a capacitance of the lumped capacitor 12. The parasitic oscillations also constitute a factor of decreasing an oscillated output power having a given resonance frequency component. Therefore, the prior art oscillator circuit of FIG. 1 using a resonance circuit formed of a microstrip line has the drawbacks that it is extremely difficult to produce oscillated signals at a prescribed frequency band ranging, for example, between 517 and 931 MHz which is demanded of an output signal from a local oscillating circuit included in a UHF television tuner, regardless of changes in the capacitance of the lumped capacitor 12; and the oscillator circuit generates a small amount of power.